Thermostatic switch



April 3, 1951 E. J. WENTWORTH THERMOSTATIC SWITCH 2 Sheets-Sheet 1 FiledMay 10, 195-3 ob Q mm. aw 3 mm an MN Inveizivfl: 122222834 J. W

April 3, 1951 E. J. WENTWORTH 2 Sheets-Sheet 2 Filed May 10, 1950 8 mI\\ QM 8Q wfi. t I R! WT E \J J m. y Iv 11! .z r lll h b I I mm a mm P aFm. mm

Patented Apr. 3, 1951 THERMOSTATIC SWITCH Elmer J. Wentworth,Framingham, Mass, assignor to Smith Control and Instrument Corporation,Needham, Mass., a corporation of Massachusetts Application May 10, 1950,Serial No. 161,106

5 Claims.

This invention relates to improvements in a thermostatic switch and moreparticularly to a thermostatic switch used for opening or closing anelectrical circuit responsive to temperature changes and essentially ofthe type which constitutes a self-contained unit and may be described asthe tubular type of a thermostatic switch.

A thermostatic switch of the character to which this invention relatesessentially comprises a casing or container generally tubular in formand strut members within the case fixed to the opposite ends of thecase. The strut members are made of material having a relatively lowcoefficient of expansion as compared with that of the case. Thelongitudinal expansion or contraction of the case produces relativemovement between the strut members. The electrical circuit connectionsto the case lead to a pair of electrical contacts within the case. Therelative movement between the strut members is effective throughsuitable operating mechanism to open or close the electrical circuit.Resilient means, such as leaf spring members, must also be used tonormally bias the operating means either to close the circuit or to holdthe circuit open. Suitable adjusting means is also desirable to presetthe switch by loading the resilient means so that the electrical circuitwill be either opened or closed as desired at the pre-determinedtemperature.

Thermostatic switches of this general type are disclosed in the patentgranted to Norman J. Smith 2,441,725, issued May 18, 1948 and in thepending application of Norman J. Smith, Serial No. 120,631, filedOctober 11, 1949. In the Smith patent, No. 2,441,725, a resilientleaf-spring member forms a part of the strut. It is desirable to makethe strut of Invar or other material which has a very low rate ofthermo-expansion. On the other hand, a spring member which will retainover a long period of time a fixed degree of resiliency can not be madefrom Invar. The spring element should be made of Inconel or other highgrade of spring steel.

It is an object of my invention to provide a thermostatic switch of thetype above-described in which the strut members are made of Invar andthe resilient member does not form an operating part of a strut andtherefore can be made of Inconel.

It is a further object of my invention to provide a pair of strutmembers fixed to the opposite ends of the case and a lever memberextending transversely to the struts which is moved by the relativemovement between the free ends of the struts and a spring actingindependently of the struts to provide a resilient load upon the levermember.

A still further object of my invention resides in the construction whichpermits ready assembly of the operating parts, which, it should beunderstood, are made of relatively small size. The strut member and theleaf-spring member have overlapping ends formed with openings in whichmay be inserted the T-shaped head of the lever member. The assembly ofthe lever member thereby serves to lock the ends of the struts and theleaf spring while permitting relative sliding movement between the endsof the struts and the spring member.

Further objects and advantages of my improvements will be more readilyapparent from the following description of a preferred embodimentthereof as illustrated in the accompanying drawings: in which,

Figure 1 is a longitudinal section taken through the thermostatic switchand shown on an enlarged scale;

Figure 2 is an elevation showing the head end of the switch in Figure 1;

Figure 3 is a fragmentary horizontal section taken on the planeindicated 3-3 in Fig. 1;

Figure 4 is a horizontal section of the thermostatic switch;

Figure 5is an exploded perspective view showing the association ofcertain of the operating parts of the thermostatic switch; and

Figure 6 is a fragmentary vertical section showing a modified assemblyof the struts and spring with respect to the lever.

Thermostatic switches of the type to which this invention relates may beconstructed to be direct acting or reverse acting. In the direct actingtype, which is the most common, the electrical contacts are normallybiased to closed position and the expansion of the case will produceopening of the contacts. In the reverse acting type, the contacts arenormally biased to open position and the expansion of the case willproduce closure of the contacts. There is a further division betweenswitches designed to undershoot or overshoot. That is, provision is madefor relative lost motion between the strut members after the contactsare closed in the undershoot form, or provision is made for lost motionin the strut members after the contacts are opened in the overshootform. The direct acting type of switch or the transverse acting type ofswitch may be made to undershoot or overshoot as desired. Thus there arefour basic types of switches to suit special operating conditions. Itwill be further understood that in my improvements only simple changesin assembly are required to make a switch of the desired type. Forexample, a compression spring or a tension spring may be used and thelost motion may be made to be effective as desired.

My improvements are directed to a thermostatic switch of a well knowntype which is usually made of relatively small size and whichessentially comprises an expansible case If! and operating parts withinthe case for opening or closing an electrical circuit. The case lllispreferably tubular in form and is .adapted to expand or contractresponsive to temperature changes. The case H) has a head 2 at one endand a bottom closure M at the opposite end. Insulating mountings it areprovided in the head for the electrical terminals [8. The inner ends 2!]of the terminals 18 are fitted with electrical contacts 22. It will,therefore, be understood that my switch does not require an insulatedwiring within the case.

The pull strut 2 is made of Invar or other material'of relatively lowco-efilcient of expansion. The strut M :is formed with an L-shaped endwhich is suitably welded and brazed to the bottom end M. A secondstrutmember 28 is provided; the end .of which overlaps the end of thestrut 24. The head l2 of the case receives a sleeve 30 into which isthreaded the adjusting bolt 32. The "bolt 32'is formed with an internalthreaded bore-3 the threads of which are preferably finer than theexterior threads of the bolt 32. A threadedmember-tfi is received in thebore 34 and has an extension 38 which is suitably welded to thestrut-member 28. The sleeve member 3!) is formed with slots 49 adaptedto receive the fingers Q2 of strut 28. The strut E8 is therefore free.to slide with respect to the sleeve member 30 but is held againsttwisting. Turning the adjusting bolt 32 in one direction to move thebolt inward with respect to the sleeve 30 will at the same time draw thescrew member 35 of strut member 28 into the bolt 38 at a slower rate,thereby adjusting the strut member 28 inwardly of the case with respectto the sleeve 30. Turning the adjusting bolt 32 in the oppositedirection will similarly adjust the strutmember 28 by moving itoutwardly with respect to the case.

The bow-shapedspring .member is is welded at one end it to the strut .24near the end 25 of the strut. The opposite end 48 of the spring 4% isflattened and rests against the underside of the strut 2s. Thespring-member i l may be made of high grade spring steeL-such asInconel, since it does not need to have a low rate of expansion relativeto the case.

Referring toFigure 5, it will be noted that the strut :28 has an opening50; the strut 24 has an opening 52 and the spring 44 has an opening Eachof these openings, 50, 52 and-54, is made of the same size and shape andit will be noted that in each-case the side edges of the openings orholes which extend'tr'ansversely of the parts are inwardly bowed. Figure5 also shows the contact lever member '55. The lever 56 has theshoulders 58 which engage the bottom surface of the end '43 of thecompression spring 34. The tongue portion 69 of the lever 55 is receivedin the three openings, 50, 52 and '56. The T-shaped head 62 of .thelever55 looks on the top surface of the strut 28 as clearly shown in Fig. i.It will be understood that the lever 56 may be inserted at an anglethrough the holes, st, 52 and 54, and

4 then turned to lock the struts 28, 24 and the spring 44 together.

The space of each of the openings, 50, 52 and St between the bowed sideedges is greater than the thickness of the neck portion of the lever 56to permit a small degree of lost motion. The lever member 56 has acircular opening 64 to fit on the insulated bushing 66 which has acircular projection fitting into the opening 54. A similar insulatedbushing 68 is held against the opposite side of the lever 56 and theseparts are assembled on a pin 10 which has a riveted head 12. A washer Mis interposed between the head 12 and the bushing Q8. The bushing 65 isengaged against the enlarged portion 76 of the pin 10. The pin memberiii also serves as a mounting for a dumbbell-shaped common contactmember 18 which is formed with two circular contact ends Elil adapted toengage the contacts 22. Thus, the contact member 18 forms a bridgebetween the contacts .22. Since the lever is free to rock in theopenings,.5ll, 52 and 5 as it rests against the bowed sides thereof, thebridging .contact 18 will make a uniform pressure contact against eachof the contacts 22.

The switch disclosed in Figure 1 is a direct acting undershoot type ofswitch. The leaf spring 34 is loaded under compression and the free end48 of the spring i l is acting to move the lever 62 in a direction tohold the dumbbell bridge contact it against the contact buttons 22. Theadjusting screw 32 may be set as indicated in Fig. 1 to oppose thethrust of the leaf spring, thus the struts 2d and 28 will be under aninitial tension load. As the case It expands under a rise intemperature, the struts 24 and 28 will not expand at the same rate andthe increased load on the struts will finally overcome the force of theleaf spring 24 and move the dumbbell contact l3 away from the contactbuttons 22. It will be understood that the engagement of the strut 28against the lever 55, as shown in Fig. 4, will act as a fulcrum point.The strut 24 will pull on the, lever at a point spaced from the fulcrumpoint and will oppose the action of the leaf spring.

The compression leaf spring it could be loaded under tension, and bysuitable adjustment of the screw the struts could be placed. under acompressive load acting to oppose'the tension of the leaf spring. Bythis setting of the switch, contraction of the case would decrease theforce of the struts tending to oppose the spring and allow the spring toopen the contacts. This would be a switch of the direct acting overshoottype.

In order to make the switch of the reverse acting type, it is onlynecessary to assemble the strut 28 between the spring 46 and the strut24 as shown in Fig. 6. In this case, the spring 24 is a tension springtending to open the contacts and. the strut 2% is acting to oppose theopening of the contacts. The strut 28 will be acting as a fulcrumbetween the spring it and the strut 24. Expansion of the case willincrease th load on the strut 24- and cause the lever 56 to rock in adirection to close the contacts. The reverse acting type of switch mayalso be made with a compression loaded spring with the struts also undera compressive load acting to oppose the spring.

I claim:

1. In a thermostatic switch, a tubular case adapted to expand orcontract responsive to 'a change in temperature, a circuit closing leverwithin said case extending transversely thereof and adapted to rocktoopen or close an electrical circuit, a short strut fixed with respectto one end of said case and adapted to form a fulcrum for said lever, asecond longer strut fixed to the opposite end of said case and adaptedto act upon said lever at a point spaced from the fulcrum of said shortstrut, said strut members having a coemcient of expansion less than thatof said case and a leaf spring within said case acting upon said leverto oppose the force of said second strut, whereby said lever will bemoved in one direction when the load on said second strut overcomes theforce exerted by said leaf spring.

2. In a thermostatic switch, a tubular case adapted to expand orcontract responsive to a change in temperature, a lever member mountedfor rockable movement within said case and extending transverselythereof, means movable by said lever to open or close an electricalcircuit leading into said case, a short strut fixed with respect to oneend of said case and adapted to form a fulcrum for said lever, a secondlonger strut fixed with respect to the opposite end of said case andarranged to act upon said lever at a point spaced from the fulcrum ofsaid short strut, said struts having cooperating openings to detachablyreceive said lever and a leaf spring acting upon said lever to opposethe force of said second strut, whereby said lever will be moved in onedirection when the load on said second strut overcomes the force exertedby said leaf spring.

3. In a thermostatic switch, a case having a head closure at one end anda bottom closure at the opposite end, a pair of electrical circuitterminals mounted in the head end of the case, a pair of fixedelectrical contacts supported by said terminals within the case adjacentthe head end,

a pair of strut members supported by the opposite ends of said case,said strut members having overlapping relatively slidable ends withinthe case, the overlapping ends of said struts having openings adapted toreceive a lever member extending transversely with respect to saidstruts,

a bridge contact member supported by said lever member adapted to engagesaid pair of contacts and resilient means separate from said strutsacting upon said lever member, whereby expansion of the case willproduce relative movement between said struts to tilt said lever andmove said bridge contact member with respect to said fixed electricalcontacts.

4. In a thermostatic switch, a tubular case having a head closure at oneend, a bottom closure at the opposite end, a strut member fixed to thebottom member of said case, a second strut member adjustably mounted atthe head end of said case for relative slidable movement with respect tofirst strut, said struts having overlapping ends, a lever membersupported by the overlapping ends or" said struts and extendingtransversely with respect thereto, a pair of circuit terminals mountedin the head end of said case, contacts carried by said terminals withinsaid case, a bridge contact member supported by said lever and adaptedto engage said contacts to complete the electrical circuit and resilientmeans normally effective to move said lever in a direction to close theelectrical circuit, whereby expansion or" the case will produce relativemovement between said struts to tilt said lever and open said circuit.

5. In a thermostatic switch, a tubular case, a head closure at one endof said case and a bottom closure at the opposite end of said case, afirst strut member fixed to the bottom end of said case, a second strutmember supported from the head end of said case, said strut membershaving relatively overlapping ends, means for adjusting the position ofthe second strut member longitudinally of said case, a bow-shapedcompression spring fixed at one end adjacent the bottom end of saidcase, said spring member having its free end slidably associated withrespect to the inner ends of said struts, said struts and the free endof said compression spring having openings formed therein, a levermember having a T-shaped head adapted to be received by the openings inthe ends of said struts and said spring, said lever forming a lock tohold the ends of said struts and said spring in slidable overlappingposition and means to open and close an electrical circuit within saidcase responsive to expansion or contraction of said case producingrelative movement between said struts and tilting of said lever.

ELMER J. WENTWORTH.

No references cited.

